<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .slider-bar {
            position: absolute;
            left: 50%;
            top: 300px;
            margin-left: 600px;
            width: 45px;
            height: 130px;
            background-color: pink;
        }

        .w {
            width: 1200px;
            margin: 10px auto;
        }

        .header {
            height: 150px;
            background-color: purple;
        }

        .banner {
            height: 250px;
            background-color: skyblue;
        }

        .main {
            height: 1000px;
            background-color: yellowgreen;
        }

        span {
            display: none;
            position: absolute;
            bottom: 0;
        }
    </style>
</head>

<body>
    <div class="slider-bar">
        <span class="goBack">返回顶部</span>
    </div>
    <div class="header w">头部区域</div>
    <div class="banner w">banner区域</div>
    <div class="main w">主体部分</div>
    <script>
        var sliderbar = document.querySelector('.slider-bar');
        var banner = document.querySelector('.banner');
        var goBack = document.querySelector('.goBack');
        var bannerTop = banner.offsetTop;
        var sliderbarTop = sliderbar.offsetTop - bannerTop;
        document.addEventListener('scroll', function () {
            if (window.pageYOffset >= bannerTop) {

                sliderbar.style.position = 'fixed';
                sliderbar.style.top = bannerTop + 'px';
                goBack.style.display = 'block';

            } else {

                sliderbar.style.position = 'absolute';
            }
        })
        //点击返回顶部
        goBack.addEventListener('click', function () {
            window.scroll(0, 0)
        })
    </script>
</body>

</html>